With turbochargers for combustion engines in a power range suitable for motor vehicles, the turbine driving the compressor is fed by the exhaust gas of the combustion engine and to date has usually been constructed out of metallic alloys having high strength at high temperatures.
With reference to the increasing use of turbochargers in automobiles, an increased high temperature strength and an improved behavior of acceleration is desired. These requirements could be fulfilled by the use of ceramic materials in the turbocharger turbine. Ceramic materials, such as silicon nitride or silicon carbide, have a nearly constant high strength in the range of temperature under consideration, and have a density which is only one-third of that of a metallic material.
Due to the high temperature strength of such materials, the operating temperatures can be raised without danger, while the mass moment of inertia, due to the relative low density of the ceramic turbocharger rotor, can be reduced to about 40% of the moment of inertia of a metallic type rotor, and therefore, the time of response of the turbocharger correspondingly improves.
The experiments to develop such turbine wheels out of ceramic materials have up to now not yet led to the desired success, as there have resulted difficulties in the production process of the wheel, as well as other problems. Inadmissable defects to the components were found in the preferred, low-cost process, in which the radial turbine wheels, produced out of the ceramic material, are first of all fabricated as so-called green parts by injection molding or slip-casting techniques and afterwards subjected to a burn-out procedure for the binder followed by sintering or nitriding procedures. These defects are mainly attributed to a prevented escape of the gases produced by the burn-out of the binder as well as by a non-uniform shrinkage of the wheel with the solid body of the hub. A central, hollow bore, which would be favorable for the production process, is however not realizable for strength reasons, as by that shape the tensions resulting in operation would be increased to double the value of a hub without a bore.
The object of this invention is therefore construction of a ceramic turbine wheel, which, although it has a sufficient high temperature strength, does not show the above described difficulties in production.